1. Field of the Invention
This invention relates to optical tellurium oxide glasses and fibers for Mid Infrared (MIR) devices, and more particularly to a tellurium oxide glass composition that improves optical performance in the MIR band without sacrificing stability, strength or chemical durability.
2. Description of the Related Art
Glass and glass optical fiber have become critically important engineering materials over the last half century. Glassy materials are generally capable of transmitting light in one or more of several wavelength “bands” loosely defined as Ultraviolet (UV) from 0.01-0.39 microns, Visible (Vis) from 0.39-0.750 microns, Near Infrared (NIR) from 0.750-2.0 microns, Mid Infrared (MIR) from 2.0-5.0 microns, and Long Wave IR>>5.0 microns. The glasses and optical fibers are used in many applications including but not limited to communications, spectroscopy, chemical sensing, medical applications, generating and/or guiding laser light and other optical sources.
Generally speaking, the glasses and optical fibers may be used in linear, non-linear and active devices. In linear devices, light is transmitted without changing the properties of the light. Typical linear devices include light guiding structures such as transport fiber, single and multimode fibers and waveguides. In non-linear devices, the optical output is a nonlinear function of the intensity of the light, often resulting in creation of frequencies not present in the input. Typical non-linear devices include Raman lasers and amplifiers, optical switches and supercontinuum sources. In active devices, the glass includes an active dopant that provides optical gain. For each pump photon absorbed, the dopant re-emits multiple photons. Typical active devices include lasers and amplifiers.
To be useful, a glass must exhibit low optical losses, typically measured in dB/m, for the band or bands of interest. Absorption at wavelengths within the band produces optical loss. Furthermore, the glass and glass optical fiber must be strong, chemically durable and stable (i.e. not prone to crystallization).
In the 1970's silica glass and fiber was developed. Silica based optical fiber became the material of choice for long haul data transmission due to exceptional low loss in the so-called telecom window near 1.5 micron wavelength. Silica fiber however is limited by inherent absorption of the silica glass at wavelengths longer than about 2.3 microns.
U.S. Pat. No. 3,883,357 to Cooley proposed a laser glass composition comprising Tellurium (IV) Oxide TeO2, Lanthanum Oxide La2O3; and Zinc Oxide ZnO and an effective lasing amount of Nd2O3 for stimulated emission at a wavelength of about 1.06 microns. As stated at col. 3, lines 29-53, this glass exhibited unexpectedly high fluorescent activity at 1.06 microns and thus the potential for enhanced gain compared to silica glass. These and other Zinc Tellurite glasses did not capture a large market due to the very mature low-cost process for making high quality silica based fiber with exceptionally low loss.
In the 1990's and early 2000's tellurite glass again gained favor as the need for telecom amplifiers drove research into erbium doped tellurite lasers operating at 1.5 microns (U.S. Pat. Nos. 5,251,062; 6,266,191; 6,413,891). Tellurite glass possesses broad glass forming regions and excellent rare earth solubility as compared to silica to support higher dopant concentrations. Over the next decade these active devices moved to slightly longer wavelengths up to 2 microns by incorporating Thulium and Holmium dopants thus covering the NIR wavelength range. These glasses often suffered due to losses caused by water (hydroxyl OH—) incorporated into the glass during melting which quenches gain by these active NIR dopants.
Researchers continue to search for glasses and optical glass fiber that exhibit low loss well into the MIR band and possess the requisite glass transition temperature Tg, stability, strength and durability. MIR glass has been developed in several distinct families including halide, chalcogenide, and oxide types.
Of the possible halides including fluorine, chlorine, bromine and iodine, only fluoride-based glasses have gained some commercial use due to a severe lack of chemical durability and potential toxicity of chlorides, bromides and iodides. Fluoride glass exhibits good transmission characteristics to greater than 5 microns but has not gained widespread acceptance due to low chemical durability, low physical strength, difficulty in achieving low loss fusion splices, low melting temperatures that make them not suitable for coating with anti reflective coatings by common vapor phase methods and difficulty in routinely producing very long lengths without defects. The inherent low melting temperatures of fluoride glasses also limit laser damage thresholds and maximum average power handling capability.
Chalcogenide glass based on the elements Sulfur, Selenium and Tellurium has good transmission from the near infrared to long infrared region but does not transmit in the visible. These glasses do not contain oxygen but are made up of inter-metallic structures such as As2S3, As2Se3, GeS2, GeSe2, etc. . . . . Chalcogenides also have distinct absorption peaks in the MIR region. Chalcogenides are physically extremely weak leading to fiber breakage during manufacturing of cables as well as in use in high vibration environments. Chalcogenides possess very low melting temperatures making them not suitable for common vapor phase coating processes.
Oxide glasses based on Tellurium can theoretically transmit light with low loss to beyond 5 microns. The optical losses into the MIR are inherently higher with tellurium oxide than with either the halides or chalcogenides because of its higher phonon energy. However, known formulations of oxide glasses for NIR applications possess the required stability, strength and chemical durability lacking in the halide and chalcogenide glasses. These formulations have typically relied on the incorporation of Lead (Pb), Germanium (Ge), Tungsten (W), Niobium (Nb) and Sodium (Na) as well as various others atomic species to overcome the tendency towards crystallization. Lead being toxic is avoided when possible.
To improve optical performance of these Tellurium oxide glasses into the MIR band, researchers have directed their efforts to developing glass formulations and processing that reduce the amount of hydroxyl (OH—) that is entrapped in the glass during melting.
U.S. Patent Pub. No. 2003/0045421 to Berger describes an optical tellurite glass for waveguide amplifiers and oscillators comprising TeO2, ZnO, PbO, Nb2O5, La2O3 and/or other rare earth oxides (dopants) and metal halides that have good melting and processing properties and a high crystallization stability and a low water content. In [0031] Berger claims a “surprisingly low OH-group absorption of the glasses of less than 3.5 dB/cm at 3,200 nm.”
Liao et. al. “Preparation and characterization of new fluorotellurite glasses for photonics application”, Journal of Non-Crystalline Solids 355 (2009) 447-452 fabricated new glasses based on TeO2—ZnF2—PbO—Nb2O5 for mid-infrared lasers. The addition of ZnF2 changed significantly the glass optical properties. In particular, the absorption loss in the visible and infrared regions of the fluoro-tellurite glasses (e.g. 10 mol % ZnF2) was much reduced compared with that of the tellurite glass, which was because the hydroxide (OH) groups decreased markedly.
Jonathan Massera et al. “Processing of Tellurite-Based Glass with Low OH Content” J. Am. Ceram. Soc. 1-7, 2010 reported on the processing and characterization of tellurite-based glass in the TeO2—Bi2O3—ZnO (TBZ) glass family and specifically on efforts to reduce their absorption loss due to residual (OH) content. Massera replaced the 20 mol % ZnO of the control glass with 20 mol % ZnF2 and added a fluorinating agent NH4F—HF to reduce hydroxyl. After melt, the final glass composition includes greater than 7 mol % of Zinc Fluoride. Massera also was able to reduce hydroxyl content by performing the melt in an oxygen rich atmosphere.
Heike Ebendorff-Heidepriem et al. “Extruded tellurite glass and fibers with low OH content for mid-infrared applications” Optical Materials Express, Vol. 2, Issue 4, pp. 432-442 (2012) reports a fluoride-free process using a dry atmosphere for the glass melt that enables the absorption at the OH peak at 3.3 microns to be reduced by more than an order of magnitude compared with glasses melted in open air. They reported an OH absorption peak of 40-50 dB/m at 3.3 microns for a fluoride-free glass composition of 73 T3O2-20 ZnO-5 Na2O-2 La2O (in mole %).